Main Processes and Motility
Upon entering the stomach cavity, food begins to swell and liquefy, and its components gradually dissolve. The ingested mass is not mixed with acidic gastric juice all at once, creating conditions for different types of digestion to occur in different layers of the food bolus.
In the central part of the food contents, salivary enzymes (carbohydrases) remain active, continuing to break down carbohydrates. Since native gastric juice lacks amylolytic enzymes, this hydrolysis continues only until the acidic environment penetrates deep into the bolus and inactivates the carbohydrases.
Simultaneously, in the parietal layer (right next to the mucosa), protein digestion begins via gastric juice enzymes. This occurs provided that the acidic secretion is not neutralized by the buffering properties of the food itself. The width of the active digestion zone depends on the properties of the food and the volume of juice. As chemical processing and liquefaction proceed, contractions of the stomach wall shift the digested chyme layer to the antrum, from where it is emptied into the intestine in portions.
Gastric Juice and Secretory Cells
The secretion of the mucosal glands plays a leading role in digestion. The fundus produces juice rich in pepsins and hydrochloric acid. Conversely, the secretion of the pyloric region is poor in enzymes. The reaction of the secretion is acidic: after a meal, due to the buffering properties of food, the pH ranges from 1.8 to 4.0.
Specific gland cells are responsible for producing the juice components:
- Chief cells — synthesize inactive forms of enzymes (pepsinogens) and lipases.
- Parietal cells — secrete hydrochloric acid (HCl) and intrinsic factor.
- Mucous neck cells — secrete mucoid substances (gastric mucus).
According to its chemical composition, the juice is divided into inorganic substances (hydrochloric acid, chlorides, sulfates, phosphates, bicarbonates, sodium, potassium, calcium, and magnesium cations) and organic substances (enzymes, mucin, amino acids, urea, and uric acid).
Enzymatic Breakdown of Nutrients
The primary enzymatic process in the stomach is the initial hydrolysis of proteins into peptones and albumoses. Proteolytic enzymes are produced in an inactive form and require activation by hydrochloric acid.
There are two groups of enzyme precursors:
- Group I pepsinogens (5 types). Produced in the gastric fundus, converted into pepsins. Their pH optimum is 1.5–2.0 (maximum hydrolysis rate is achieved at this acidity).
- Group II pepsinogens (2 types). Produced in the pyloric region, converted into gastricsin. The optimum action is in a less acidic environment — pH 3.2–3.5.
Clinical significance: the presence of enzymes with varying pH optima ensures efficient protein digestion in different layers of chyme with varying acidity.
Among additional enzymes, rennin (chymosin) stands out, which curdles milk in the presence of calcium ions. Fat digestion is limited: local lipase breaks down lipids into fatty acids and glycerol, but acts only on emulsified fats.
Functions of Hydrochloric Acid and the Mucus-Bicarbonate Barrier
Hydrochloric acid performs critical tasks in gastric digestion:
- Triggers protein denaturation and swelling, facilitating their subsequent breakdown.
- Converts inactive pepsinogens into functional pepsins.
- Provides the acidic environment (pH optimum) required for the activity of proteolytic enzymes.
- Exerts a pronounced antibacterial effect, sterilizing the chyme.
- Participates in the regulation of gastrointestinal motor activity.
To prevent aggressive acid and pepsins from damaging the stomach wall, a protective mechanism exists — the mucus-bicarbonate barrier. It consists of gastric mucus (mucoids) and bicarbonate anions ($HCO_3^-$) produced along with it. This barrier coats the mucosa, protecting it from mechanical trauma and autodigestion.
Mucoids also include intrinsic factor (gastromuconin). It is critical for the intestinal absorption of vitamin $B_{12}$ (extrinsic factor). Their interaction forms the anti-anemic blood factor.